FIELD OF THE INVENTION
The invention relates to a device for printing to stock, in particular a piece of mail in postage meters and/or addressing machines.
With such devices, the stock is guided past a printing device and the postage indicia or address is printed in a single pass.
The stock is typically guided past the printing device while lying flat as is seen in U.S. Pat. No. 5,467,709, for example, or on edge as is seen in U.S. Pat. No. 5,025,386, for example.
In each case, it is important to ensure that the stock and the printing device are brought into a defined position relative to one another so that the mark is printed in the intended location and with sufficient quality.
In the case of horizontal transport of the stock, a relatively large bearing surface, corresponding to the largest stock format to be printed, is required, thus the machine has a correspondingly large footprint.
In the device disclosed in U.S. Pat. No. 5,467,709, an ink jet print head provides contactless printing. The piece of mail is fed between a driven conveyor and spring-mounted pressure rollers, whereby the piece of mail rests against a longitudinal guide plate. The longitudinal guide plate has a cutout matching the conveyor and a rectangular cutout for the ink jet print head. A row of nozzles in the print head run along the diagonal of the cutout. The conveyor, the longitudinal guide plate and the ink jet print head are located above the piece of mail. Spring-mounted pressure rollers and a spring-mounted pressure roller located in the print area are located below the piece of mail.
The spring travel of the pressure rollers and the pressure plate corresponds to the maximum thickness of the piece of mail, which can vary between 2 mm and 20 mm.
The spring force must be appropriate for the entire range of weights of pieces of mail, approximately 20 to 1000 g, and must also ensure that the piece of mail is held sufficiently planar in the area of the cutout for the print head.
Contactless ink jet printing requires that the smallest possible distance be maintained between the stock and the ink jet print head. That both minimizes the effects of inaccurate ink spray and prevents the stock from contacting the nozzle surface, thus preventing smearing.
However, there is still a risk of smearing when the piece of mail leaves the area of the cutout and inevitably glides along the longitudinal guide plate.
These conditions are difficult to maintain when rapidly processing pieces of mail of varying dimensions. In other words, the rapid processing does not allow the ink to dry prior to further transport following the imprint.
The prior art also discloses a postage meter, as is seen in U.S. Pat. No. 5,025,386, in which the piece of mail is carried on edge and slightly inclined on a rotating conveyor. The pieces of mail rest against a guide block which has a print window. A thermal print head with which the postage indicia is printed on the piece of mail can be moved laterally and vertically within the print window.
The size of the print window must be adapted to the maximum length and width of the printed image.
The individual piece of mail is transported to the print window, then stopped and pressed through the use of a pressure plate against the guide plate or the print window. The pressure plate is driven by a motor through a toothed gearing and crankshaft. It is a relatively complex mechanism and significant counterpressure must also be provided for thermal printing.
After printing, the piece of mail is released and transported away. It is clear that only a low throughput is possible with such an intermittent mode of operation. Positioning of the thermal print head is complex.